adders

Basking and feeding

After emergence in the spring, the adders need to bask and feed. Basking is done to absorb heat and regenerate vitamins. An adder can only be active once its temperature has reached a certain minimum value, and so its ability to feed is dependent on the weather.

You might expect sometimes to see a snake with a swelling after it has swallowed a small animal. You might wait a long time. By the time that a small mammal has had the air squeezed from its lungs, had its rib-cage squashed, and had its fur pressed down to its body, it doesn’t occupy anything like the volume that the live animal seems to occupy. If the prey is a small lizard, you certainly won’t see much sign of it. We must remember that a great many animals take smaller prey than we might imagine. We may think of foxes in terms of chickens and lambs, but much of the time they have to make do with things like grubbed up beetles. In the case of a snake, small invertebrates are taken. Adders do not need to eat many times during the summer, because they live rather inactive lives.

Contrast this with the life of the hyperactive shrew, which must eat voraciously just to remain alive. So although there are great benefits from being a mammal, there can be disadvantages too. In a sense no living animal is more "advanced" than any other, as we have all survived as species to the present day. Adders must have many genes in common with us, because they have many of the same organs (greatly elongated and narrowed), and they have descended, like us, from four-legged animals. We must in fact share a common ancestor with them. We have some concerns in common – we have to survive, eat and reproduce. Nevertheless the adder is an alien creature. We cannot share its world.

No mammals, even those who live underground, or seek prey which live underground, have become as long and thin as snakes. One penalty paid by snakes for this shape is that the organs are long and thin, and undoubtedly less efficient than those of mammals. For a mammal, such a shape would result in a much greater rate of loss of heat for a given mass, and the animal would need to carry more fat, carry more fur, or eat more, in order to maintain temperature, at least in a cold climate. The first two methods would tend to negate the thinness, and the last is an obvious penalty. Mustelids such as the stoat and the weasel seem to be about as far as the mammals have gone in this direction. In fact, on average, the smaller a mammal, or the colder its surroundings, the more nearly spherical it needs to be to conserve heat. Is this borne out by the facts?

A snake swallows its prey whole, because it is not equipped with cutting teeth or chewing teeth, though it may possess small backward pointing ones. By advancing different parts of its mouth in turn, the snake slowly engulfs the prey. By taking the prey head first, the snake uses the the ratchet effect of the fur, as well as the ratchet effect of the teeth, to help in moving the animal back. The teeth are not permanent: the snake drops them and grows new ones throughout its life.

Some snakes can find a mammal by detecting the infra-red radiation emitted by the warm body. Of course, all objects emit this radiation all the time: the snake detects the difference between the animal’s emission and that from the surroundings. Although the temperature difference is small, the rate of emission is a strong function of temperature, varying as the fourth power of the temperature. A difference in temperature of 5 Celsius represents about 1.7 per cent on the absolute scale. The relative difference in radiation emission might be four times as much as the relative temperature difference, about 7 per cent in the example given.

Adders, however, use their forked tongue to detect substances being carried in the air. The tongue is being used for the purpose that mammals use their noses.

When the adder is close enough to the prey, it strikes with the fangs, which are at the front of the mouth, injecting a lethal dose of venom down a canal in each fang. The venom does not kill immediately, but the animal soon weakens and dies. The adder has only to follow the scent and the warmth to find the dead animal. Snake venoms are quite complicated liquids, containing digestive enzymes as well as the venom. The enzymes can greatly hasten decomposition of tissue. Many of the venomous constituents work by interfering with nervous impulses; some degree of paralysis may result, as well as difficulty in breathing. The venom may impede coagulation of the blood, and may affect the cardio-vascular system.

The fangs of some snakes have grooves to carry the venom, but in vipers the grooves have closed over to form a canal or tube.

The female adder produces live young, and towards the end of pregnancy, she may be seriously hampered in feeding. The energy loss is so great that females breed no more often than alternate years, and sometimes less. In a bad year, the female might even not bring the development of the young to completion, and they will be born during the next year.

More information about basking and temperature control can be found by clicking here and here.